Method of anodically polishing zinc



April 10, 1945. L, FAUST v 2,373,466

METHOD OF ANODICALLY POLISHING ZINC Filed Nov. 6, 1959 mofqgo Q CHARLESL. FAIAST chromic acid and Patented Apr. 10, 1945 2,373,466 METHOD OFANODICALLY POLISHING ZINC Charles L. Faust, Columbus, hio,'assignor,toBattelle Memorial Institute, Columbus, Ohio, a

corporation of Ohio Application November 6, 1939, Serial No. 302,954

4 (llaims.

This invention relates to a method of and'an electrolyte for anodicallypolishing zinc and zinc alloys. More particularly the invention relatesto an aqueous electrolyte comprising phosphoric acid, chromic acid andwater and to the use of such electrolyte in the anodic polishing ofzinc.

I have now found that an aqueous electrolyte comprising phosphoric acid,water, as the essential ingredients, can be satisfactorily used in theanodic polishing of zinc, zinc base dies casting alloys, zincalloys-commonly used in extrusion processes and, in general, any Zincalloy containing more than 50% of zinc. If the relative proportions ofthe three ingredients of the bath be kept within certain limits, which Ihave determined, very satisfactory polishes can be produced in the caseof zinc, while at the same time obtaining surfaces having relativelysuperior characteristics to those prepared by mechanical polishing orbufling operations Best results are obtained on zinc itself, or onalloys of zinc containing over 90% of zinc.

It is therefore an important object of my invention to provide anelectrolyte containing phosphoric acid, chromic acid and water' withincertain limits as to their relative proportions, for use in the anodicpolishing of zinc and zinc alloys to impart thereto a highly lustroussurface. I

It is a further important object of this invention to provide a methodof anodically polishing zinc and zinc alloys using an electrolyte ofnovel composition that is operative throughout a wide range of anodecurrent densities and temperatures.

Other and further important objects of' this invention-willbecomeapparent from the disclosures in the specification and theaccompanying drawing.

This invention (in its preferred form) is illustrated in the drawing andhereinafter more fully described.

On the drawing the figure represent a triaxial diagram showing therelative phosphoric acid, chromic acid and water in compositions ofelectrolyte coming within the scope of my invention.

In the accompanyingdiagram, the respective chromic acid and proportionsof v the lines AB, BE,

tive compositions of electrolyte is defined on the accompanying diagramby thesolid lines AB, BC, CD, and IDA. Within the area so defined, anycomposition selected will be found to be operative in the methodhereinafter described for the anodic polishing of zinc.

In order to get the best polishing results, however, I have found thatthe range of proportionsof phosphoric acid, chromic acid and watershould be kept'within somewhat narrower limits and these narrower limitsare represented on the accompanyin diagram by the area defined by dotand dash lines EF', and FA. The preferred compositions of electrolyte,with respect to the relative proportions of phosphoric acid, chromicacid and water, lie within this second lesser area, which is whollyenclosed within the broader area first'deflned.

The reading of a triaxial diagram such as the accompanying one followingwill be given for purposes of illustration. The point on the diagramrepresented by the letter A, for instance, indicates a compositioncomprising a small but significant proportion, say 0.1% of phosphoricacid, 65% of chromic acid and the balance, somewhat less than 35%,water; the point indicated by the reference letter B a compositioncomprising 85% of phosphoric acid, say about 0.1% of chromic acid andthe balance, somewhat less than 15%, water; the point indicated by thereference letter C a composition comprising 32% phosphoric acid, sayabout 0.1% of chromic acid and the balance somewhat less than 68%,water: and the point represented by the reference letter D a compositioncomprising say about 0.1% of phosphoric acid, 49% of chromic acid andthe balance, somewhat less than 51%, water.

It will be seen that the maximum phosphoric acid content is 85% and theminimum about 0.1%, the maximum chromic acid'content 65% sides of thetriangle indicate the percentages of phosphoric acid (1131 04), of water(H20), and of chromic acid (CrOa), from 0 to 100%. On the basis ofexperimental data, I have determined the relative proportions ofphosphoric acid, water that give compositions of are operative for theanodic The area representing operaelectrolyte that polishing of zinc.

and the minimum about 0.1%; the maximum water content about 68% and theminimum 13%.

The preferred composition limits are from 0.1 to 85%- phosphoric acid,from 0.1 to CrOa ,and from 13 to 56% water. The relative proportions ofphosphoric acid, chromic acid and water. however, are inter-dependent,so that instead of selecting avbath composition having percentageswithin the maximum and minimum ranges for the three ingredients, thecomposit on of electroi "lyte should be selected by reference to thetriaxi'al diagram and a point selected within the preferred area if bestresults are to be obtained.

While the accompanying triaxial diagram indi- -is well understood, butthe electropolishing of zinc,

a Notwithstanding such cates relative percentages of phosphoric acid,

mic acid and water should bear to each other may be determined foroperative and preferred'ranges, of composition, regardless of what othernonessential ingredients may be present in the bath.

Thus, for instance, in making up an electrolyte for the anodic polishingof zinc, one might select the point X on the triaxial diagram as being abath of preferred composition. The composition represented by vthe pointX would be 60% phosphoric acid, chromic acid and 30% water. During thecontinued use of such a bath in the the bath composition wouldnecessarily change, owing to the anodic dissolution into the bath ofzinc. There might also be some change in the water content, such as anincrease due to absorption by the bath of moisture from the air, or adecrease due to evaporation of water from the bath, to decomposition ofwater by electrolytic action, or to loss of water from the bath byentrainment thereof in gases given off from the bath.

changes in its composition as may occur during continued use, if therelative percentages of phosphoric acid, chromic acid and water,expressed as percentages by weight of thetotal weight of only thesethree ingredients in the bath composition, remain within the areadefined by the lines AB, BE, EF and PA, the bath will continue tooperate satisfactorily. Even if the bath'composition be so alteredduring continued operation that it falls within the less preferred areadefined by the lines FE, EC, CD and DF, on the accompanying dia gram,the bath will continue to function,.although not so satisfactorily.

Consequently, where the relative percentages of phosphoric acid, chromicacid and water in a given bath composition lie within either thepreferred or less preferred areas defined on'the triaxial diagram, suchbath composition is intended to come within the scope of my invention,even r though it may contain other acids than phosphoric and chromic andeven though it may contain a substantial quantity of metallic salInstead of chromic acid, soluble chromates and bichromates may besubstituted therefor and are to be considered the equivalent of chromicacid on a stoichiometric basis. acid equivalent, as used in thisspecification and in the claims, is therefore intended to includechromic acid itself (C103) and stoichiometrically equivalent weights ofsoluble chromates and bichromates.

Similarly, in place of orthophosphoric acid, other phosphoric acids,such as metaand pyrophosphoric acids, may be used and are to beconsidered as included within the term phosphoric acid."

In the method of anodically polishing zinc, using a bath of acomposition indicated as suitable by reference to the accompanyingtriaxial dia gram, the zinc, or an article having a surface of zinc, ismade the anode in a bath of the selected composition and an electriccurrent is passed therethrough of sufilcient density and for asufficient length of time to produce the desired high degree of luster,or polish, on the surface the The term" chromic also upon the characterof lying within zinc. By employing an electrolyte having a. compositionwithin the preferred area defined on the accompanying triaxial diagram,a highly lustrous, mirror-like surface can be readily obtained. Thehighly lustrous surface obtainable by my method, using an electrolyte ofpreferred composition, is an important feature of my invention and onethat sharply finishes produced in the metals.

The formation of highly polished and lustrous surfaces is undoubtedlyassociated with the presence of a polarizing film over the surface ofthe metal during the process of anodic dissolution.

distinguishes it from prior art electrolytic cleaning of .The nature ofthis film is such that selective at- In order to obtain the best resultsin a reasonable length of time, it is preferable to use controlledvoltages of l to 15 volts, at which the currenta density will suitablyadjust itself to a value within the range of 3 to 150 amperes per sq.ft. It will be found that higher densities can be used in baths of highphosphoric-chromic acid ratio,'for example up to 500 amperes per sq.ft., for which the voltage will be from 20 to 30 volts. Higher currentdensities may be employed but arenecessarily more costly because of thehigher voltages required and the consequent higher cost of electricalenergy. The length of time to effect the desired results depends uponthe magnitude of the current densities employed and to some extent uponthe particular characteristics of the zinc, or zinc alloy, to bepolished, and v thesurface of the zinc initially. Rough surfaces, ofcourse, require a longer time to polish than relatively smooth ones.

With any of the compositions of electrolyte lying within the preferredarea defined by the lines AB, BE, Eli and FA, on the accompanyingtriaxial diagram, excellent polishes are obtained upon zinc, when thezinc, or a zinc coated article, is made the anode therein at a voltageof 3 volts and the treatment is carried out for a period of 1 to 12minutes, employing a bath temperature the range of 60 to F. The currentdensity will adjust itself towithin the range of 4 to 100 amps/sq. ft.depending upon the HaPOr-CIOa ratio in the bath. In general thetemperature should not exceed F.

From the foregoing description of my invention, it will be apparent thatI have provided a novel composition of electrolyte and a method wherebythe same may be used to produce highly lustrous polishes on zinc andzinc alloys high in zinc content. My electrolyte and methodavoid thedisadvantages of the old methods of mechanical polishing and enable theproduction of zinc, and alloys of zinc, such as zinc base die castings,extruded zinc articles and the like, with surfaces that are free frommechanical strain, dragging and piling, and'which are superior to thoseobtainable by mechanical polishing methods.

It will, of course, be understood that various details ofthe process maybe varied through a wide range without departing from the principles ofthis invention and it is, therefore, not the purpose to limit thepatentgranted hereon othertherethrough of sumcient density and for asumwise than necessitated by the scope of the appended claims.

I claim as my invention:

1. The method of anodically polishing an article having a surface ofmetal selected from the group consisting of zinc and alloys thereofcontaining more than 90% zinc, which comprises making the article theanode in an aqueous solution comprising essentially 1'I3PQ4, Cr03,'andwater, the percentage proportions by weight of said three solutioningredients lying within the area defined in the accompanying diagram bythe line AB, the straight line BC, the line CD and the straight line DA,and at a temperature not in excess of 125 F., passing an electriccurrent therethrough of sumo-lent density and for a sufficient durationof time to eifect a polish on said surface.

2. The method of anodically polishing an article having a surface ofmetal selected from the group consisting of zinc and alloys thereofcontaining more than 90% zinc, which comprises making the article theanode in an aqueous solution comprising essentially HaPO4, CrOa, andwater, the percentag proportions by weight of said three solutioningredients lying within the area defined on the accompanying diagram bythe line AB, the straight line BE, the line EF, and the straight line FAand at a temperature not in excess of 125 F., passing an electriccurrent cient duration of time to eifect a polish on said surface.

3. The method of anodically polishing an article havlnga surface ofmetal selected from the group consisting of zinc and alloys thereofcontaining more than 90% zinc, which comprises making the article theanode in an aqueous solution comprising essentially H3PO4, CI'Os, andwaterjthe percentage proportions by weight of said three solutioningredients lying within the area defined on the accompanying diagram bythe line AB, the straight line BE, the line EF and the straight line FA,and passing an electric current therethrough at a voltageof from 1 to 15volts at a solution temperature of from to 100 F.

4. The method of anodically polishing an article having a surface ofmetal selected from the group consisting of zinc and alloys thereofcontaining more than zinc, which comprises making the article the anodein an aqueous solution comprising essentially H3PO4, CrOs, and water,the percentage proportions by weight of said three solution ingredientslying within the area defined on the accompanying diagram by the lineAB, the straight line BC, the line CD and the straight line DA, andpassing an electric current therethrough at avoltage of from 1 to 15volts CHARLES L. FAUST.

